1. Field of the Invention
The present invention relates to a coated optical fiber for telecommunication having improved transmission performances.
2. Description of the Related Art
An optical fiber as drawn has some drawbacks such as insufficient mechanical strength and poor maintenance of transmission characteristics. Therefore, the drawn optical fiber is coated with, for example, a resin. In general, the optical fiber has one or two coating layers. FIG. 1 shows a cross section of an optical fiber having a two-layer coating comprising a soft cushioning layer 2 which is coated around a glass optical fiber 1 and a rigid protective layer 3 surrounding the cushioning layer 2.
FIG. 2 shows a cross section of an optical fiber having a single protective layer 3. An example of this type optical fiber is an optical fiber which is coated with a UV curable resin and used in a submarine cable which requires large strength. Recently, as techniques for increasing the strength and improving heat resistance of the optical fiber are developed, highly heat resistant resins with large Young's modulus such as ladder type silicone resins or polyimide resins are used to form the protective layer.
When the resin is coated on the optical fiber as drawn, a volume of the coating layer decreases as the coated resin is cured irrespective of the UV curable resin or thermosetting resins such as ladder type silicone resins. The volume decrease of the coated layer starts at a moment when the resin is changed from a liquid state to the solid state by heating or irradiation. That is, the volume decrease starts and finishes as soon as the resin curing starts. When the volume decrease of the coated layer compresses or expands the glass fiber in a radius direction of the fiber with large force, the glass fiber is strained, so that light transmission loss at room temperature increases. Since the increase of transmission loss is significant when the protective layer 3 is directly formed on the glass fiber or it has large Young's modulus, prevention of the increase of transmission loss of the coated layer is one of the problems to be solved for increasing the strength of the optical fiber.
To prevent the increase of transmission loss, various measures have been proposed in connection with photocurable resins. For example, Japanese Patent Kokai Publication No. 54206/1987 proposes the use of a resin having a large light absorption coefficient to increase curing properties at a surface, whereby the shrinkage of the resin in the radial direction is suppressed. Japanese Patent Kokai Publication No. 95510/1987 proposes a coated optical fiber having a coating layer made of a resin with a degree of curing shrinkage of 0 (zero).
However, the above prior arts intend to decrease the compressing or expanding force in the radial direction to zero by selecting the special resins. The selection of such resins does not satisfy the requirement for physical properties, particularly high speed curing, of a coating material of the optical fiber.
According to Japanese Patent Kokai Publication No. 95510/1987, since all the disclosed non-shrinkable resins are curable resins based on ring-opening polymerization, they have a very low curing rate in comparison to radical polymerized resins having terminal vinyl groups which are widely used now.
According to Japanese Patent Kokai Publication No. 54240/1987, since most of the irradiated light is absorbed by the resin, the resin may be undercured. In particular, in case of the optical fiber having a large coating thickness, it is impossible to increase productivity. In addition, the measure disclosed in this Publication can be applied only to the photocurable resins but not to thermosetting resins.